Remove remaining unsafe or ForeignPtr-based code from FFI.gc

[sethfowler]

There are still a few types in FFI.gc that are manipulated unsafely. Before pushing anything to Hackage everything needs to be cleaned up.

[sethfowler]

Refactoring to use the phantom type parameter everywhere fixed most of these issues. UnsavedFile was the last place where we used ForeignPtr's for manual marshaling. We do still have a couple of references to ForeignPtr's in the code because Storable Vectors are implemented with ForeignPtr's internally, but we now let ByteStrings and Vectors take care of the real work.

I'd say this issue is fixed at this point.